This invention relates to a travelling wave optical modulator and particularly, but not exclusively, to a modulator for broad-band operation. By optical is meant that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides.
Guided-wave modulators which exploit the electro-optic effect are finding many applications in optical transmission and processing systems. Lithium niobate integrated optical devices are attractive, for example, as they require low drive voltages and can be fabricated by planar processes.
The electro-optic effect in semiconductor and dielectric waveguides is relatively weak, and so in order to operate efficiently at high modulation frequencies above one or two gigahertz a travelling wave structure, as opposed to a lumped electrode, is usually employed. That is, the electrode is designed as a transmission line matched to a microwave modulating input signal with the modulating signal fed colinearly with the propagating optical wave.
The bandwidth of a basic travelling wave device is limited by any mismatch between the velocities of the modulating signal propagating down the electrode and the optical signal propagating down the waveguide which causes "walk-off" of the two signals. If the velocities are equal the electrode length can be arbitrarily long allowing a low drive voltage to achieve the required phase change. However, some important materials such as lithium niobate have an inherent mismatch in these velocities so any attempt to reduce the drive voltage and power by increasing the length of the device will decreases the maximum possible drive frequency. One approach to overcoming this limitation, discussed in a paper entitled "Velocity--Matching Techniques for Integrated Optic Travelling Wave Switch/Modulators" by R. C. Alferness, S. K. Korotky and E. A. J. Marcatilli, IEEE Jul Qu Elec Vol QE-20 No. March 1984, is velocity matching by intermittent interaction. The electrode is split apart from the optical waveguide before or at a point where the velocity mismatch between the modulating signal and the optical signal would result in sufficient `walk-off` of the signals to cause a polarity reversal of the applied field. The electrode is returned to rejoin the waveguide at a re-joining point, the electrode and waveguide path having lengths between the splitting and rejoining point designed so that the optical signal in the waveguide is advanced relative to the electrode signal sufficiently to once again be subject to induced phase shifts over the next section of coincident electrode and waveguide that are in phase with those induced in the previous section. This is repeated to obtain the total effect on the phase of the optical signal required.
A disadvantage of this known intermittent interaction, velocity-matching technique is that although it provides a method of compensating the velocity-mismatch at a given predetermined, high frequency it produces no substantial increase in band width over that of a velocity-mismatch limited travelling-wave optical modulator and the accumulated phase shift generally displays a complicated dependence on the applied, modulating signal frequency. Further, it cannot operate with non-periodic modulating signals as the optical signal is modulated by successively advanced portions of the modulating signal as it progresses from one section to the next.
An alternative approach to providing intermittent action is to compensate for the walk-off by designing the optical and electrode paths such that the optical signal is retarded relative to the electrode signal to achieve the required phase matching for the subsequent interaction. Previously implementations have relied on bulk crystals with parallel reflectors to reflect a freely propagating optical signal along a zig-zag path to pass intermittently within an interaction region formed by a shorter zig-zag or straight electrode; see U.S. Pat. No. 3,791,718 and U.S. Pat. No. 3,393,954 respectively. Such known intermittent interaction optical modulators are, not however, suited to forming to fully integrated optical circuits, require careful preparation and handling of the reflective surfaces and accurate alignment of the light input to ensure the optical signal propagates in the required interaction region.
According to the present invention a travelling wave optical modulator comprises a substrate, an optical waveguide defined within the boundaries of a surface of the substrate; a travelling wave electrode by which a modulated electric field of a predetermined frequency is applicable to at least a portion of the optical waveguide; and one or more delay portions between a respective splitting point and rejoining point where the optical waveguide is separated from the electrode and for which the transit time between the splitting point and rejoining point is greater for an optical signal in the optical waveguide than for the electric field in the electrode.
With this arrangement of intermittent interaction the optical signal is delayed relative to the electrode signal to compensate for the accrued walk-off but, in contrast to known intermittent interaction devices, it is returned to rejoin the electrode so as to be modulated by the same electrode signal. The electrode signal therefore need not be periodic.